(1) Field of the Invention
The present invention relates to a molecularly oriented molded body of an ultra-high-molecular-weight ethylene/.alpha.-olefin copolymer. More particularly, the present invention relates to a molecularly oriented molded body, especially a fiber, of an ultra-high-molecular-weight ethylene/.alpha.-olefin copolymer, which has novel crystal fusion characteristics and is excellent in heat resistance and creep resistance.
(2) Description of the Prior Art
It is known that a molecularly oriented molded body having high elastic modulus and high tensile strength is prepared by molding ultra-high-molecular-weight polyethylene into a fiber, a tape or the like and drawing the molded product. For example, Japanese Patent Application Laid-Open Specification No. 15408/81 discloses a process comprising spinning a dilute solution of ultra-high-molecular-weight polyethylene and drawing the obtained filaments. Furthermore, Japanese Patent Application Laid-Open Specification No. 130313/84 discloses a process comprising melt-kneading ultra-high-molecular-weight polyethylene with a wax, extruding the kneaded mixture, cooling and solidifying the extrudate and drawing the solidified extrudate. Furthermore, Japanese Patent Application Laid-Open Specification No. 187614/84 discloses a process comprising extruding the above-mentioned melt-kneaded mixture, drafting the extrudate, then cooling and solidifying the extrudate and drawing the solidified extrudate.
When ultra-high-molecular-weight polyethylene is molded into the form of a fiber and the fiber is strongly drawn, the elastic modulus and tensile strength are increased with increase of the draw ratio, and the obtained fiber has such excellent mechanical properties as high elastic modulus and high tensile strength and are excellent in the light weight characteristic, water resistance and weather-ability. However, the heat resistance is inevitably restricted because the melting point of polyethylene is relatively low and generally in the range of from 120.degree. to 140.degree. C. Furthermore, when a fiber of ultra-high-molecular-weight polyethylene is used at a high temperature, the strength retention ratio is drastically reduced and the creep is extraordinarily increased.